The present invention relates generally to machine systems capable of being precisely positioned with respect to a workpiece and, more particularly, to machine systems having retroreflectors for permitting optical endpoint control and associated methods.
Modem machine systems oftentimes include robots, machine tools or other mechanical positioning devices, including computer numerical control (CNC) machines, (hereinafter collectively referred to as xe2x80x9cmachinesxe2x80x9d) that must be precisely positioned with respect to a workpiece in order to appropriately machine the workpiece. For example, the CNC machines employed during the manufacture of aircraft, automobiles or other vehicles must be precisely positioned with respect to the workpiece such that the resulting parts are within the relatively strict tolerances demanded by the particular application. In order to determine the position of a machine tool or a portion of a machine tool, such as the end effector of a machine tool, some machine systems include optical end point control. See, for example, U.. Pat. No. 5,903,459 which issued May 11, 1999 to Thomas A. Greenwood, et al., and which describes a precision measuring system and method, the contents of which are incorporated by reference herein. See also, U.S. patent application Ser. No. 08/867,857 filed Jun. 3, 1997 by Thomas A. Greenwood, et al., which also describes a precision measuring system and method, the contents of which are incorporated by reference herein.
Machine systems that utilize optical endpoint control include one or more retroreflectors which are mounted upon the machine to serve as targets. For example, one or more retroreflectors can be mounted upon the end effector of a machine. Machine systems that include optical endpoint control also include a laser measurement system, i.e., a laser tracker, for illuminating the retroreflectors. By detecting the light reflected by each retroreflector, the laser measurement system can determine the distance and the direction to the retroreflector. Based upon the distance of the retroreflector and the direction to the retroreflector, the position of the retroreflector and, in turn, the position of the portion of the machine that is carrying the retroreflector can be precisely determined. Based upon the determination of the actual position of the machine, the machine system can accommodate any differences that are detected between the anticipated position of the machine and the actual position of the machine. These differences in position are attributable to a wide variety of factors including expansion and contraction of the machine and the workpiece as a result of thermal changes in the factory and mechanical misalignments of and between individual axes of the machine tool. By compensating for differences between the anticipated and actual positions of the machine, the machine system can fabricate the resulting part in a much more precise and repeatable fashion.
A variety of retroreflectors have been developed to receive incident light and to reflect the light in a direction substantially parallel to the incident light. However, conventional reflectors have a relatively limited field of view known as an acceptance angle that significantly limits the applications in which retroreflectors can serve to properly reflect incident light. More particularly, light received by a retroreflector within the acceptance angle will be properly reflected by the retroreflector. However, light outside of the acceptance angle will not be reflected and, therefore, cannot be utilized to position the retroreflector. As such, the acceptance angle defined by a retroreflector restricts the position and orientation of the retroreflector relative to the light source, such as the laser tracker. This limitation is particularly disadvantageous in applications in which the retroreflector is mounted upon a machine, such as a robot or other machine tool, that can move in multiple directions and about multiple axes relative to the light source and may frequently be positioned such that the retroreflector does not face the light source, thereby preventing the light emitted by the light source from falling within the acceptance angle defined by the reflectometer. Without adding additional light sources and/or additional retroreflectors which would, in turn, increase the complexity and cost of the machine system, the position of the machine cannot therefore be determined in instances in which the incident light does not fall within the acceptance angle defined by the retroreflector, i.e., in instances in which the retroreflector does not face the light source.
Various types of retroreflectors have been developed, although each defines a relatively limited acceptance angle. One common retroreflector is a trihedral prism reflector that is frequently referred to as a solid corner cube retroreflector. The trihedral prism retroreflector has three mutually orthogonal surfaces such that light incident upon the prism is reflected generally parallel to, but laterally displaced from the incident light. While trihedral prisms are relatively inexpensive and are fairly accurate with the incident and reflected beams being parallel to within 2.0 microradians, the lateral displacement of the reflected beam from the incident beam varies due to refraction based upon the angle at which the incident light strikes the retroreflector, i.e., the incidence angle. In order to maintain accurate retroreflector properties, the trihedral prism retroreflector is therefore limited to an acceptance angle of about +/xe2x88x9215xc2x0.
Another type of retroreflector is a hollow corner cube retroreflector that is constructed of three mutually orthogonal mirrors. Although the lateral displacement between the incident and reflected beams does not vary as a function of the incidence angle, a hollow corner cube retroreflector is generally relatively difficult to manufacture and is accordingly more expensive than a comparable trihedral prism reflector. In addition, hollow corner cube retroreflectors typically have an acceptance angle of +/xe2x88x9225xc2x0.
A third type of retroreflector is a cat eye in which several hemispherical lenses are bonded to form a single optical element. While a cat eye has a larger acceptance angle, such as about +/xe2x88x9260xc2x0, a cat eye is significantly more expensive than a trihedral prism retroreflector or a hollow corner cube retroreflector. While a cat eye has a much greater acceptance angle than a trihedral prism retroreflector or a hollow corner cube retroreflector, the acceptance angle of a cat eye is still insufficient in many situations, particularly in many high precision manufacturing operations in which the retroreflector will be mounted upon the end effector of a robot or other machine tool that will assume many different positions during the manufacturing process.
One attempt to overcome the limited acceptance angles of conventional retroreflectors is to group a plurality of hollow corner cube retroreflectors in a cluster. Unfortunately, the clustered retroreflectors do not form a single, large, continuous acceptance angle. Instead, the clustered retroreflectors form a plurality of distinct acceptance angles with gaps between each acceptance angle. As such, certain angular regions still do not fall within the acceptance angle of any of the clustered retroreflectors. In addition, clustered retroreflectors have not been able to be constructed so as to simulate a single target since the retroreflectors have not been able to be positioned such that their apexes are coincident.
Accordingly, although machine systems having optical endpoint control have been developed, these machine systems are limited by the somewhat restricted acceptance angles of conventional retroreflectors which prevent the machine system from measuring the position of the retroreflector when the machine has assumed a position in which the retroreflector cannot be illuminated within its acceptance angle. In this regard, although a variety of retroreflectors are available, these conventional retroreflectors do not define acceptance angles that are sufficiently large and continuous as required by some applications. For example, retroreflectors that are mounted upon the end effector of a robot or other machine tool would preferably have an extremely large acceptance angle since the retroreflectors will be moved through a wide range of positions during typical machining operations. As such, in order to have the capability of precisely determining the position of a machine as the machine assumes a variety of positions such that the machine can be driven to compensate for positional inaccuracies and to form parts with precise dimensions, a need still remains for a machine system having retroreflectors that define a much larger acceptance angle than conventional retroreflectors.
A machine system having optical endpoint control and an associated method for monitoring the position of a machine are provided which includes at least one steerable retroreflective system that defines a relatively large effective acceptance angle, typically exceeding 320xc2x0. The area outside the acceptance angle is therefore a conic subtending an angle that is generally less than 40xc2x0. As such, the machine system and method can determine the position of the retroreflector and, in turn, the position of the portion of the machine upon which the retroreflector is mounted, such as the end effector of the machine, throughout the course of a machine operation even as the machine moves in different directions and rotates about different axes since the light which illuminates the retroreflector will generally fall within the relatively large acceptance angle defined by the retroreflector.
In one embodiment, the machine system includes a machine capable of movement in at least one direction and a steerable retroreflective system mounted upon the machine, such as upon the end effector of the machine, for movement therewith. The machine system also includes at least one light source for illuminating the retroreflector such that the position of at least a portion of the machine is determinable based upon reflections from the retroreflector. As such, the machine system and, in particular, the process controller or the machine controller that directs the movement of the machine can make any necessary corrections in the computer control of the machine to accommodate for differences between the anticipated position of the machine and the actual position of the machine.
The steerable retroreflective system includes a retroreflector for reflecting at least some light that is incident thereupon, as well as means for controllably steering the retroreflector. In one embodiment, the means for controllably steering the retroreflector includes at least one positioner, such as a motor, and a controller for directing the at least one positioner to controllably steer or move the retroreflector. For example, the controller may operate in a closed loop mode in order to direct the retroreflector to continuously follow the incident light beam provided by the light source. Alternatively, in an embodiment in which the at least one light source includes a plurality of light sources, the controller may operate in an open loop fashion so as to direct the retroreflector toward the incident light beam emitted by different respective light sources.
Although the steerable retroreflective system can include various types of retroreflectors, the retroreflector of one advantageous embodiment is a trihedral prism. The trihedral prism has an input surface through which incident light is received and a plurality of reflective surfaces for reflecting the incident light. The trihedral prism also defines at least one partially transmissive window opposite the input surface so that leakage light passes through the window and escapes from the trihedral prism.
In this regard, the steerable retroreflective system can also include an optical detector for detecting the leakage light that passes through the retroreflector. Based upon the leakage light detected by the optical detector, the controller can then direct the at least one positioner to controllably steer the retroreflector as desired. For example, the optical detector can define a target zone and the means for controllably steering the retroreflector can steer the retroreflector to move the leakage light toward the target zone.
Although the machine system need only include a single steerable retroreflective system, the machine system of one embodiment includes a plurality of individually steerable retroreflective systems. According to this embodiment, the machine system can also include a process controller for directing the at least one light source to sequentially point toward the retroreflectors of different respective retroreflective systems. As such, the machine system of this embodiment can determine the respective positions of several different portions of a machine based upon reflections from the retroreflectors mounted upon the different portions of the machine.
In operation, the method of monitoring the position of the machine illuminates a retroreflector with incident light and detects reflections of the light from the retroreflector. Based upon the reflected light, the position of the machine can be determined. According to the present invention, the retroreflector that is mounted upon the machine is also steered such that the retroreflector points toward the incident light. As such, the retroreflector can be steered so as to follow or track the incident light, thereby significantly increasing the effective acceptance angle of the retroreflector. Based upon the actual position of the machine as determined from the reflected light, the movement of the machine can be more precisely directed since the machine system can compensate for any differences between the anticipated position of the machine and the actual position of the machine as determined from the reflected light.
While the machine system and associated method of one embodiment includes at least one steerable retroreflective system mounted upon the machine, the steerable retroreflective system can, instead, be mounted upon the workpiece. As such, the position of the workpiece can be precisely determined by illuminating the retroreflector on the workpiece and by detecting the light reflected by the retroreflector. By controllably steering the retroreflector, such as based upon the leakage light passing through the retroreflector and detected by an optical detector, the retroreflector of this embodiment also has a larger effective acceptance angle. According to one aspect of this embodiment of the invention, the light source can be mounted upon the machine and adapted to move therewith. As such, the light reflected from the retroreflector mounted upon the workpiece will provide an indication of the relative spacing and position of the machine and the workpiece. According to another aspect of this embodiment of the invention, the light source can be remote from the machine and the workpiece, but retroreflectors can be mounted upon both the machine and the workpiece. As such, illumination of the retroreflectors mounted upon the machine and the workpiece permits the relative positions of the machine and the workpiece to be precisely determined.
By controllably steering the retroreflector, the retroreflector can effectively have an extremely large acceptance angle. For example, the retroreflector can have an acceptance angle that typically exceeds 320xc2x0. As such, the area outside the acceptance angle is a conic subtending an angle that is generally less than 40xc2x0. This design permits light to be received by the retroreflector from a much wider range of angles than conventional retroreflectors. As such, the machine system and associated method of the present invention can illuminate the retroreflector and detect reflections therefrom even as the machine moves in various directions and rotates about different axes since the retroreflector can be correspondingly steered to continuously face the light source. Thus, the movement of the machine can be more precisely directed since the actual position of the machine can be tracked during most, if not all, of the machine operations and compensation can be provided for any differences between the anticipated position of the machine and the actual position of the machine.